Sexual and parasexual genetic analyses have permitted the detection, characterization and genetic localization of a number of mammalian cellular genes which participate in neoplastic transformation and retroviral expression. Seven classes of genes are under study: (1) endogenous cellular DNA sequences homologous to retroviral RNA; (2) integration sites of retroviruses in mammalian chromosomes; (3) growth factor receptors; (4) restriction genes which delimit viral replication; (5) enzyme structural genes; (6) cell surface antigens including major histocompatibility (MHC); and (7) cellular transforming genes (onc) rescued by defective retroviruses. Examples of each of these gene classes have been detected and mapped in human and feline model systems. Somatic cell genetic procedures have been used to derive a biochemical genetic map of the domestic cat of 35 genes distributed on 19 cat chromosomes. An extraordinary degree of linkage conservation between the cat and man has been demonstrated. The cat, which has previously been shown to be an excellent model for leukemia, sarcoma and a variety of inborn human errors, now is the third major mammal (after man and mouse) suitable for genetic analysis.